Method of using a surfactant-containing shrinkage material to prevent photoresist pattern collapse caused by capillary forces

ABSTRACT

A first photoresist pattern and a second photoresist pattern are formed over a substrate. The first photoresist pattern is separated from the second photoresist pattern by a gap. A chemical mixture is coated on the first and second photoresist patterns. The chemical mixture contains a chemical material and surfactant particles mixed into the chemical material. The chemical mixture fills the gap. A baking process is performed on the first and second photoresist patterns, the baking process causing the gap to shrink. At least some surfactant particles are disposed at sidewall boundaries of the gap. A developing process is performed on the first and second photoresist patterns. The developing process removes the chemical mixture in the gap and over the photoresist patterns. The surfactant particles disposed at sidewall boundaries of the gap reduce a capillary effect during the developing process.

PRIORITY DATA

The present application is a continuation of U.S. application Ser. No.15/062,956, filed Mar. 7, 2016, now U.S. Pat. No. 10,090,357 whichclaims benefit of provisional patent application 62/272,127, filed onDec. 29, 2015, the contents of which are hereby incorporated byreference herein in their entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. However, these advances haveincreased the complexity of processing and manufacturing ICs and, forthese advances to be realized, similar developments in IC processing andmanufacturing are needed. In the course of integrated circuit evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased.

The ever-shrinking geometry size brings challenges to semiconductorfabrication. For example, photoresist masks may be more prone to theeffects of capillary forces. This is exacerbated as the aspect ratio ofthe mask increases and/or as the pitch decreases. As a result,photoresist masks may collapse, for example due to being pulled bycapillary forces between adjacent photoresist masks.

Therefore, while existing semiconductor fabrication technologies havebeen generally adequate for their intended purposes, they have not beenentirely satisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 illustrates surfactant particles being mixed in a chemicalmaterial according to embodiments of the present disclosure.

FIGS. 2-15 are diagrammatic fragmentary cross-sectional side views of asemiconductor device at various stages of fabrication in accordance withembodiments of the present disclosure.

FIGS. 16-18 are flowcharts illustrating methods of fabricating asemiconductor device in accordance with embodiments of the presentdisclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the formation of a first feature over or on a second feature in thedescription that follows may include embodiments in which the first andsecond features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. Various features may be arbitrarily drawn indifferent scales for the sake of simplicity and clarity.

As semiconductor fabrication technologies continue to evolve, the devicesizes are becoming increasingly small. When the device sizes aresufficiently small, capillary forces may adversely interfere withfabrication. For example, the formation of photoresist masks may involvea developing process using a developer solution. At a sub-micron pitchlevel, the developer solution used in the developing process may causethe collapse of photoresist patterns due to capillary forces thateffectively “pull” on the photoresist patterns. This problem isexacerbated when photoresist patterns are formed to have a tall andnarrow (or high aspect ratio) trench disposed in between. The tall andnarrow trench means that the photoresist mask is even more prone to theeffects of capillary forces, which makes the collapse of the photoresistpatterns more likely. Therefore, some conventional fabricationtechniques have reduced the aspect ratio of the trench, for example bylowering the height of the photoresist patterns or increasing theirseparation. However, the resulting photoresist mask may not be capableof meeting the demands of advanced semiconductor fabrication processes,which may need the tall and narrow photoresist masks to pattern contactholes or define radiation-sensing pixels for image sensors.

To overcome the problems discussed above, the present disclosureinvolves a method of forming photoresist masks that are less prone tothe capillary forces without compromising the aspect ratio, as discussedbelow with reference to FIGS. 1-18.

FIG. 1 illustrates a mixing process in which surfactant particles orsurfactant compounds are mixed into a chemical. In more detail, acontainer 50 contains a chemical material 60. In some embodiments, thechemical material 60 includes a “resolution enhancement lithographyassisted by chemical shrinkage” material (or RELACS). The RELACSmaterial includes a water-soluble material (e.g., a polymer) havingthermal cross-linking properties. As such, a portion of the RELACSmaterial coated on a photoresist film can become cross-linked to thephotoresist film during a baking process, thereby reducing gaps betweenadjacent photoresist films. The rest of the unreacted (e.g.,un-cross-linked) RELACS material can be removed in a developing processfollowing the baking. As examples, the details of the RELACS materialare discussed in an article entitled “Resists Join the Sub-LambdaRevolution,” by Laura J. Peters, published in SemiconductorInternational, in September, 1999, as well as in Japanese PatentApplication KOKAI publication No. H10-73927, the contents of each ofwhich are hereby incorporated by reference in their respectiveentireties.

In some other embodiments, the chemical material 60 includes a “shrinkassist film for enhanced resolution” material (or SAFIER) developed byTokyo Ohka Kogyo Co. The SAFIER material includes an aqueous solutionthat contains thermo-responsive polymers that facilitate photoresistflow during a baking process. The SAFIER material may or may not reactwith the photoresist chemically but provides mechanical support to thesidewalls of the photoresist as it flows. The mechanical supportprovided by the SAFIER material minimizes photoresist pattern profiledegradation. The SAFIER material can be removed in a developing processfollowing the baking. As an example, the details of the SAFIER materialare discussed in a paper entitled “Electron-beam SAFIER™ process and itsapplication for magnetic thin-film heads,” by XiaoMin Yang, et al.,published in the Journal of Vacuum Science & Technology B, Volume 22,Issue 6, in December 2004, the contents of which are hereby incorporatedby reference in its entirety.

According to the various aspects of the present disclosure, surfactantparticles 70 are mixed in the chemical material 60. The surfactantparticles 70 include compounds or molecules that lower the surfacetension (or interfacial tension) between liquids or between a liquid anda solid. For example, a surfactant particle may include a moleculehaving one end that is water-soluble and an opposite end that isoil-soluble. The surfactant molecules may aggregate to form micelles. Insome embodiments, the surfactant particles 70 each include a fluorinatedcompound. In some other embodiments, the surfactant particles 70 eachinclude a hydrocarbon compound.

As is shown in FIG. 1, the surfactant particles 70 are mixed in thechemical material 60 such that they are homogeneously distributedthroughout the chemical material 60. In other words, the surfactantparticles 70 are mixed uniformly or evenly within the chemical material60. In some other embodiments, the surfactant particles 70 are stilldistributed throughout the chemical material 60, but the distributionmay not be entirely uniform. In any case, the chemical mixture obtainedby mixing the surfactant particles in the chemical material 60 will beapplied to photoresist patterns in a later process. For ease ofreference, this chemical mixture may be interchangeably referred to as ashrinkage material hereinafter.

FIGS. 2-15 are diagrammatic fragmentary cross-sectional side views of asemiconductor device undergoing various stages of fabrication accordingto embodiments of the present disclosure. Referring to FIG. 2, asubstrate 100 is provided. The substrate 100 may include a siliconsubstrate that is doped with an n-type dopant such as phosphorous orarsenic, or it may be doped with a p-type dopant such as boron. Thesubstrate 100 could also include other elementary semiconductors such asgermanium and diamond. The substrate could optionally include a compoundsemiconductor and/or an alloy semiconductor. Furthermore, the substrate100 could include an epitaxial layer (epi layer), may be strained forperformance enhancement, and may include a silicon-on-insulator (SOI)structure. It is also understood that one or more layers may also beformed over the substrate 100, but these layers are not specificallyillustrated in FIG. 2 for reasons of simplicity. For the purposes of thepresent disclosure, the reference to the substrate 100 hereinafter maybe considered to include just the substrate 100 itself, or it may beconsidered to include the one or more layers formed over the substrate100.

Photoresist patterns 110 and 111 are formed over the substrate 100. Thephotoresist patterns 110 and 111 may be formed by depositing (e.g.,spin-coating) a photoresist film over the substrate 100 and thereafterpatterning the photoresist film in a lithography process, which mayinvolve one or more processes such as exposure, post-exposure bake,developing, etc. (not necessarily in that order). The photoresistpatterns 110-111 shown in FIG. 2 are at an after-developing-inspection(ADI) stage. At this stage, the photoresist patterns 110-111 areseparated by a gap 120, or it may be said that the photoresist patterns110-111 define a trench 120. The gap/trench 120 has a height 130(vertical dimension) and a width 140 (lateral dimension). An aspectratio of the trench 120 may be defined as a ratio of the height 130 andthe width 140.

As semiconductor device fabrication technologies progress, it is desiredto have a greater aspect ratio. One way to increase the aspect ratio isto reduce the width 140 by application of the shrinkage material. Forexample, referring to FIG. 3, the shrinkage material 60 is applied overthe substrate 100 and coated on the photoresist patterns 110-111. In theembodiment shown in FIG. 3, the shrinkage material 60 includes theRELACS material discussed above. The coating of the shrinkage material60 may be done using a spin-coating process. As discussed above, thesurfactant particles 70 have been mixed throughout the shrinkagematerial 60, for example in a homogeneously distributed manner. As such,the surfactant particles 70 are mixed throughout the coated shrinkage 60(i.e., coated on and around the photoresist patterns 110-111) as well.Thus, there are surfactant particles 60 disposed on (or near) thesidewall surfaces of the photoresist patterns 110-111.

Referring now to FIG. 4, a baking process (or heating process) 180 isperformed to the shrinkage material 60 and the photoresist patterns110-111. In some embodiments, the baking process is performed at aprocess temperature in a range between about 140 degrees Celsius and 170degrees Celsius and for a process duration in a range between about 60seconds to about 120 seconds. The baking process 180 forms cross-linkingfilms 200 and 201 around the photoresist patterns 110-111. In responseto the thermal energy in the baking process 180, portion of theshrinkage material 60 (RELACS material herein) coated around thephotoresist patterns 110-111 undergo a chemical reaction, therebycausing these portions of the shrinkage material 60 to becomecross-linked. As a result, cross-linking films 200-201 are formed on theupper and sidewall surfaces of the photoresist patterns 110-111. Due tothe cross-linking nature, the cross-linking films 200-201 adhere to theupper and sidewall surfaces of the photoresist patterns 110-111, whichprevents the cross-linking films 200-201 from being removed in asubsequent developing process. In other words, the cross-linking films200-201 may be viewed as a part of the enlarged photoresist patterns110-111 when the photoresist patterns 110-111 are used as masks insubsequent processes.

According to the various aspects of the present disclosure, at leastsome of the surfactant particles 70 are distributed on (or near) thesidewall surfaces of the cross-linking films 200-201, for example on ornear the sidewall surfaces 220-221. As discussed above, the surfactantparticles 70 are configured to reduce surface tension on the sidewallsurfaces (e.g., the surface tension on the sidewall surfaces 220-221).As such, the surfactant particles 70 disposed on the sidewall surfacesof the cross-linking films 200-201 can reduce the capillary forcesexperienced by the photoresist patterns 110-111 in a developing processperformed later, as discussed below in more detail.

Referring now to FIG. 5, a developing process 250 is performed to removethe portions of the shrinkage material 60 that are not cross-linked yet.The developing process 250 includes applying a developer solution torinse the shrinkage material 60 and the photoresist patterns 110-111. Insome embodiments, the developer solution contains pure water ordi-ionized water (DIW). In other embodiments, the developer solutioncontains tetramethyl ammonium hydroxide (TMAH). The removal of theshrinkage material 60 forms a trench (or gap) 120A between thephotoresist patterns 110-111. Compared to the previous trench 120 shownin FIG. 2, the reduced trench 120A has an increased height (verticaldimension) 130A and a reduced width (lateral dimension) 140A. Thereduced width 140A allows smaller device sizes to be achieved. Forexample, the photoresist patterns 110-111 may be used (as a mask) topattern a smaller contact hole as a part of an interconnect structure,or they may be used (as an ion implantation mask) to form a smallerpixel for an image sensor device.

As discussed above, the surfactant particles 70 that are disposed on ornear the sidewall surfaces 220-221 reduce the surface tension of thesidewall surfaces. Capillary force is correlated with (or is a functionof) the surface tension. For example, Fc (capillary force) is correlatedwith γ*cos 2θ (surface tension). Since the surface tension on thesidewalls 220-221 is reduced by the presence of the surfactant particles70 disposed thereon, the capillary forces are reduced inside the trench120A as well.

The reduction of the capillary forces inside the trench is beneficial,because it reduces the likelihood of the photoresist patterns 110-111collapsing. In more detail, as the device sizes become small, the trench120A is becoming narrower and narrower. Consequently, the effects ofcapillary forces inside the trench 120A become more and more pronounced.The capillary forces effectively “pull” the photoresist patterns 110-111toward each other. If the capillary forces are stronger than theadhesion of the photoresist patterns 110-111 to the substrate 100, thenone (or both) of the photoresist patterns 110-111 may topple over andcollapse. The likelihood of the photoresist patterns 110-111 collapsingis further increased as the trench 120A becomes taller and narrower. Inother words, since the trench 120A has a smaller width 140A and greaterheight 130A than the trench 120, its aspect ratio is increased, whichmakes the photoresist patterns 110-111 otherwise more likely tocollapse.

However, the presence of the surfactant particles 70 on the sidewallsurfaces 220-221 reduce the surface tension on the sidewalls, therebyreducing the capillary forces experienced by the photoresist patterns110-111 during the developing process 250. As such, even though thetrench 120A has an increased aspect ratio, the reduction of thecapillary forces in turn reduces the likelihood of the photoresistpattern collapse. This means that the photoresist patterns 110-111 canbe formed to be taller and closer together (i.e., having a higher aspectratio trench in between) than traditionally feasible. For example,whereas the aspect ratio of the trench between photoresist patterns canhave an aspect ratio as high as about 8:1 or 9:1 without riskingphotoresist pattern collapse, the trench 120A can be formed to have anaspect ratio as high as 11:1, or as high as 12:1 (or even greater)without risking photoresist pattern collapse. The high aspect ratio(i.e., the taller and more closely located) photoresist patterns 110-111are advantageous in performing subsequent fabrication processes such ascontact hole etching or pixel formation by ion implantation, which willbe discussed in greater detail further below.

FIGS. 3-5 illustrate an embodiment of the present disclosure where theRELACS material is used as the shrinkage material 60. FIGS. 6-8illustrate another embodiment of the present disclosure where the SAFIERmaterial (discussed above with reference to FIG. 1) is used as theshrinkage material. For reasons of clarity and consistency, similarcomponents appearing in FIGS. 3-8 will be labeled the same.

Referring to FIG. 6, the shrinkage material 60 that contains the SAFIERmaterial is applied over the substrate 100 and coated on the photoresistpatterns 110-111. Again, since the surfactant particles 70 have beenmixed throughout (e.g., homogeneously distributed or uniformlydistributed) the shrinkage material 60, the surfactant particles 70 aredisposed on (or near) the sidewall surfaces of the photoresist patterns110-111 as well.

Referring now to FIG. 7, a baking process (or heating process) 180 isperformed to the shrinkage material 60 and the photoresist patterns110-111. In some embodiments, the baking process is performed at aprocess temperature in a range between about 100 degrees Celsius and 160degrees Celsius and for a process duration in a range between about 60seconds to about 90 seconds. The SAFIER material containsthermo-responsive polymers that facilitate a photoresist flow during thebaking process 180. In other words, the photoresist patterns 110 and 111flow outward laterally and are reshaped as photoresist patterns 110A and111A, respectively. The sidewalls of the photoresist patterns before theflow occurs are illustrated herein as the broken lines, and thedirections of the flow are illustrated using the arrows that pointlaterally in FIG. 7. The sidewalls 220-221 of the photoresist patternsconsequently move closer toward each other, thereby reducing thedistance in between the photoresist patterns. The height of thephotoresist patterns 110A-111A is also reduced due to the lateralexpansion. The shrinkage material 60 (i.e., the SAFIER material) alsoprovides some mechanical support to the sidewalls 220-221 of thephotoresist patterns 110A-111A during the photoresist flow, therebyallowing the sidewalls 220-221 to maintain their shapes.

After the photoresist flow forms the photoresist patterns 110A-111A, atleast some of the surfactant particles 70 are still distributed on (ornear) the sidewall surfaces 220-221 of the photoresist patterns110A-111A, since the surfactant particles are mixed homogeneously withinthe shrinkage material 60. As discussed above, the surfactant particles70 are configured to reduce surface tension on the sidewall surfaces220-221, which reduces the capillary forces experienced by thephotoresist patterns 110A-111A in a developing process discussed below.

Referring now to FIG. 8, a developing process 250 is performed to removethe shrinkage material. The developing process 250 includes applying adeveloper solution to rinse the shrinkage material 60 and thephotoresist patterns 110-111. In some embodiments, the developersolution contains di-ionized water (DIW). The removal of the shrinkagematerial 60 forms a trench (or gap) 120B between the photoresistpatterns 110A-111A. Compared to the previous trench 120 shown in FIG. 2,the reduced trench 120B has a reduced height (vertical dimension) 130Band a reduced width (lateral dimension) 140B. The reduced width 140Ballows smaller device sizes to be formed. For example, the photoresistpatterns 110A-111A may be used (as a mask) to pattern a smaller contacthole as a part of an interconnect structure, or they may be used (as anion implantation mask) to form a smaller pixel for an image sensordevice. The reduced height 130B can be compensated by forming tallerphotoresist patterns 110-111 in the first place.

As discussed above, the surfactant particles 70 that are disposed on ornear the sidewall surfaces 220-221 reduce the surface tension of thesidewall surfaces. Also as discussed above, capillary force iscorrelated with the surface tension. Since the surface tension on thesidewalls 220-221 is reduced by the presence of the surfactant particles70 disposed thereon, the capillary forces are reduced inside the trench120B as well. And for the reasons discussed above, the reduction of thecapillary forces inside the trench 120B reduces the likelihood of thephotoresist patterns 110-111 collapsing. This means that the photoresistpatterns 110A-111A can be formed to be taller and closer together (i.e.,having a higher aspect ratio trench such as greater than 11:1) thantraditionally feasible, which is advantageous in subsequent fabricationprocesses such as contact hole etching or pixel formation by ionimplantation, which are discussed in greater detail below.

FIGS. 9-11 illustrate a process of forming conductive contacts using thephotoresist patterns 110-111 of FIG. 5 or the photoresist patterns110A-111A of FIG. 8 as a mask. Referring to FIG. 9, a source/drainregion 300 is formed in the substrate 100. The source/drain region 300is a source or drain of a transistor device, such as a MOSFET device.The source/drain region 300 may be formed by one or more ionimplantation or diffusion processes. A dielectric layer 310 is formedover the source/drain region 300. The dielectric layer 310 may contain alow-k dielectric material.

The photoresist patterns 110-111 of FIG. 5 (or the photoresist patterns110A-111 of FIG. 8) are formed over the dielectric layer 310 by theprocesses discussed above in association with FIG. 2-5 or 6-8. In otherwords, the photoresist patterns 110-111 (or 110A-111A) are formed bypatterning a photoresist film into different photoresist patternsseparated by a gap/trench 120A (or 120B), and then reducing thatgap/trench with the application of a shrinkage material 60 with thesurfactant particles 70 mixed therein. The reduced trench 120A/120B canachieve a higher aspect ratio (i.e., narrower and/or taller) due to thesurfactant particles on its sidewalls that reduce capillary forces asthe shrinkage material 60 is removed during a developing process. Forreasons of simplicity, the surfactant particles 70 (or the cross-linkedfilm 200-201) are not specifically illustrated in FIG. 9. Thephotoresist patterns 110-111 (or 110A-111A) may now be used as a mask topattern a contact hole for the source/drain region 300.

Referring now to FIG. 10, an etching process 330 is performed to thedielectric layer 310 to extend the trench 120A/120B into the dielectriclayer 310. In other words, the dielectric layer 310 is “opened”, so asto expose a portion of the source/drain region 300. This “opening” isalso referred to as a contact hole, since a conductive contact is to beformed in there subsequently. The etching process 330 is performed usingthe photoresist patterns 110-111 (or the photoresist patterns 110A-111A)as an etching mask. The greater aspect ratio of the trench 120A (or120B) achieved by the processes of the present disclosure allows thiscontact hole to be formed smaller (e.g., narrower). Again, this can bedone without risking the collapse of the photoresist patterns 110-111(or 110A-111A).

Referring now to FIG. 11, the photoresist patterns 110-111 (or110A-111A) are removed, for example through a photoresist ashing orstripping process. A conductive contact 350 is formed in the contacthole, which may be performed by depositing a conductive material such astungsten (or copper or aluminum) in the contact hole. The conductivecontact 350 provides electrical connectivity to the source/drain region300 of the transistor.

In addition to forming contacts (i.e., forming contact holes) fortransistors, the processes of the present disclosure may also be used inthe fabrication of an image sensor device. The image sensor device is asemiconductor image sensor configured to sense radiation such as light.Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) andcharge-coupled device (CCD) sensors are widely used in variousapplications such as digital still camera or mobile phone cameraapplications. These devices utilize an array of pixels in a substrate,including photodiodes and transistors, that can absorb radiationprojected toward the substrate and convert the sensed radiation intoelectrical signals. The image sensor device further includes additionalcircuitry and input/outputs that are provided adjacent to the grid ofpixels for providing an operation environment for the pixels and forsupporting external communication with the pixels.

The image sensor device may be a front side illustrated (FSI) imagesensor or a back side illuminated (BSI) image sensor. In the embodimentillustrated in FIG. 12, a back side illuminated image sensor device isused, but it is understood that the aspects of the present disclosuremay apply to the front side illuminated image sensor as well.

Referring to FIG. 12, the back side illuminated image sensor device 400includes a substrate 100, which may also be referred to as a devicesubstrate. The device substrate 100 has a front side (also referred toas a front surface) 410 and a back side (also referred to as a backsurface) 420. For a BSI image sensor device such as the image sensordevice 400, radiation is projected from the back side 420 (after asubstrate thinning down process discussed below) and enters theremaining substrate through the back surface 420. In some embodiments,an initial thickness of the substrate is in a range from about 100microns to about 3000 microns, for example between about 500 microns andabout 1000 microns.

The photoresist patterns 110-111 of FIG. 5 (or the photoresist patterns110A-111 of FIG. 8) are formed over the substrate 100 by the processesdiscussed above in association with FIG. 2-5 or 6-8. In other words, thephotoresist patterns 110-111 (or 110A-111A) are formed by patterning aphotoresist film into different photoresist patterns separated by agap/trench 120A (or 120B), and then reducing that gap/trench with theapplication of a shrinkage material 60 with the surfactant particles 70mixed therein. The reduced trench 120A/120B can achieve a higher aspectratio (i.e., narrower and/or taller) due to the surfactant particles onits sidewalls that reduce capillary forces as the shrinkage material 60is removed during a developing process. For reasons of simplicity, thesurfactant particles 70 (or the cross-linked film 200-201) are notspecifically illustrated in FIG. 12.

The photoresist patterns 110-111 (or 110A-111A) may be used as a mask inan ion implantation process 440 performed to implant dopant ions intothe front side 410 of the substrate 100. The photoresist mask preventsions from being implanted into regions of the substrate 100 disposedtherebelow. The ions are implanted through the trench 120A (or 120B)into the substrate. The implanted ions have an opposite type ofconductivity than the substrate 100 and form a radiation-sensing element450 of the image sensor device 400, for example as a part of the pixelto detect light that is projected toward the substrate from the backside 420. In some embodiments, the radiation-sensing element 450 is apart of a photodiode. Although a single radiation-sensing element 450 isshown in FIG. 12, it is understood that a plurality of radiation-sensingelements are formed in a similar manner, which may be referred to as apixel array.

As discussed above, the trench 120A (or 120B) is formed to have agreater aspect ratio than the trenches in conventional processes. Again,this greater aspect ratio can be achieved without risking the collapseof the photoresist patterns 110-111 (or 110A-111A). The smaller width140A (or 140B) of the trench 120A (or 120B) allows the width (lateraldimension) of the radiation-sensing element 450 to be smaller comparedto conventional radiation-sensing elements too, since the width of theradiation-sensing element 450 is directly correlated with the width ofthe trench 120A (or 120B). The smaller width of the radiation-sensingelement 450 allows a greater number of pixels to be packed onto the samesize package for the image sensor device 400, or allows the image sensordevice 400 to be smaller than conventional image sensor devices, both ofwhich are considered improvements. In addition, the taller photoresistpatterns 110-111 (or 110A-111A) allow the photoresist mask to be moreeffective at blocking dopant ions from being implanted into the wrongregions of the substrate 100. In other words, the photoresist mask ismore effective due to its increased height.

Although the embodiment in FIG. 12 uses the photoresist patterns 110-111(or 110A-111A) as an ion implantation mask directly, it is understoodthat in alternative embodiments, the photoresist mask may be used topattern a layer therebelow to form a hard mask first, and the hard maskis then used as the ion implantation mask in the ion implantationprocess 440.

Additional processes are performed to complete the fabrication of theimage sensor device 400. Referring to FIG. 13, the photoresist mask isremoved. An interconnect structure 480 is formed over the front side 410of the device substrate 100. The interconnect structure 480 includes aplurality of patterned dielectric layers and conductive layers thatprovide interconnections (e.g., wiring) between the various dopedfeatures, circuitry, and input/output of the image sensor device 400.The interconnect structure 480 includes an interlayer dielectric (ILD)and a multilayer interconnect (MLI) structure. The MLI structureincludes contacts, vias and metal lines. The MLI structure may includeconductive materials such as aluminum, aluminum/silicon/copper alloy,titanium, titanium nitride, tungsten, polysilicon, metal silicide, orcombinations thereof, being referred to as aluminum interconnects. Theinterconnect elements may be formed by a process including physicalvapor deposition (PVD) (or sputtering), chemical vapor deposition (CVD),atomic layer deposition (ALD), or combinations thereof. Othermanufacturing techniques to form the aluminum interconnect may includephotolithography processing and etching to pattern the conductivematerials for vertical connection (e.g., vias/contacts) and horizontalconnection (e.g., conductive lines).

Still referring to FIG. 13, a buffer layer 490 is formed on theinterconnect structure 480. In the present embodiment, the buffer layer490 includes a dielectric material such as silicon oxide. Alternatively,the buffer layer 490 may optionally include silicon nitride. The bufferlayer 490 is formed by CVD, PVD, or other suitable techniques. Thebuffer layer 490 is planarized to form a smooth surface by a CMPprocess.

Thereafter, a carrier substrate 500 is bonded with the device substrate100 through the buffer layer 490, so that processing of the back side420 of the device substrate 100 can be performed. The carrier substrate500 in the present embodiment is similar to the substrate 100 andincludes a silicon material. Alternatively, the carrier substrate 500may include a glass substrate or another suitable material. The carriersubstrate 500 may be bonded to the device substrate 100 by molecularforces—a technique known as direct bonding or optical fusion bonding—orby other bonding techniques known in the art, such as metal diffusion oranodic bonding.

The buffer layer 490 provides electrical isolation between the devicesubstrate 100 and the carrier substrate 500. The carrier substrate 500provides protection for the various features formed on the front side410 of the device substrate 100, such as the radiation-sensing element450. The carrier substrate 500 also provides mechanical strength andsupport for processing of the back side 420 of the device substrate 100as discussed below. After bonding, the device substrate 100 and thecarrier substrate 500 may optionally be annealed to enhance bondingstrength.

Referring to FIG. 14, after the carrier substrate 500 is bonded, athinning process 520 is then performed to thin the device substrate 100from the backside 420. The thinning process 520 may include a mechanicalgrinding process and a chemical thinning process. A substantial amountof substrate material may be first removed from the device substrate 100during the mechanical grinding process. Afterwards, the chemicalthinning process may apply an etching chemical to the back side 420 ofthe device substrate 100 to further thin the device substrate 100 to athickness on the order of a few microns. In some embodiments, thethickness of the thinned substrate 100 is greater than about 1 micronbut less than about 5 microns. It is also understood that the particularthicknesses disclosed in the present disclosure are mere examples andthat other thicknesses may be implemented depending on the type ofapplication and design requirements of the image sensor device 400.

Referring now to FIG. 15, a color filter layer 540 may be formed on theback side 420 of the substrate 100. The color filter layer 540 maycontain a plurality of color filters that are positioned such that theincoming radiation is directed thereon and therethrough. The colorfilters may include a dye-based (or pigment based) polymer or resin forfiltering a specific wavelength band of the incoming radiation, whichcorresponds to a color spectrum (e.g., red, green, and blue).Thereafter, a micro-lens layer 550 containing a plurality ofmicro-lenses is formed over the color filter layer 540. The micro-lensesdirect and focus the incoming radiation toward specificradiation-sensing regions in the device substrate 100, such as theradiation-sensing element 450. The micro-lenses may be positioned invarious arrangements and have various shapes depending on a refractiveindex of a material used for the micro-lens and distance from a sensorsurface. The device substrate 100 may also undergo an optional laserannealing process before the forming of the color filter layer 540 orthe micro-lens layer 550.

It is understood that the sequence of the fabrication processesdescribed above is not intended to be limiting. Some of the layers ordevices may be formed according to different processing sequences inother embodiments than what is shown herein. Furthermore, some otherlayers may be formed but are not illustrated herein for the sake ofsimplicity. For example, an anti-reflection coating (ARC) layer may beformed over the back side 420 of the substrate 100 before the formationof the color filter layer 540 and/or the micro-lens layer 550.

It is also understood that the discussions above pertain mostly to apixel array region of the image sensor device 400. In addition to thepixel region, the image sensor device 400 also includes a peripheryregion, a bonding pad region, and a scribe line region. The peripheryregion may include devices that need to be kept optically dark. Thesedevices may include digital devices, such as application-specificintegrated circuit (ASIC) devices or system-on-chip (SOC) devices, orreference pixels used to establish a baseline of an intensity of lightfor the image sensor device 400. The bonding pad region is reserved forthe formation of bonding pads, so that electrical connections betweenthe image sensor device 400 and external devices may be established. Thescribe line region includes a region that separates one semiconductordie from an adjacent semiconductor die. The scribe line region is cuttherethrough in a later fabrication process to separate adjacent diesbefore the dies are packaged and sold as integrated circuit chips. Forthe sake of simplicity, the details of these other regions of the imagesensor device 400 are not illustrated or described herein.

The above discussions also pertain to a BSI image sensor device.However, it is contemplated that the various aspects of the presentdisclosure may be applied to a front side illuminated (FSI) image sensordevice as well. For example, the FSI image sensor device also usespixels similar to the pixels 210 discussed herein to detect light,though the light is projected (and enters the substrate) from the frontside, rather than the back side. The FSI image sensor device does notinvolve wafer back side thinning processes, and will instead form thecolor filters and micro-lenses on the front side. The interconnectstructure is implemented in a manner so as to not impede or obstruct thepath of incident light projected from the front side. It can be seenthat the photoresist patterns formed in accordance with the presentdisclosure (with a high aspect ratio trench) can be used to form theradiation-sensing elements for the FSI image sensor device as well.

FIG. 16 is a flowchart illustrating a method 600 of fabricating asemiconductor device according to embodiments of the present disclosure.The method 600 includes a step 610 of forming a first photoresistpattern and a second photoresist pattern over a substrate. The firstphotoresist pattern is separated from the second photoresist pattern bya gap.

The method 600 includes a step 620 of coating a chemical mixture on thefirst and second photoresist patterns. The chemical mixture contains achemical material and surfactant particles mixed into the chemicalmaterial. The chemical mixture fills the gap.

The method 600 includes a step 630 of performing a baking process on thefirst and second photoresist patterns. The baking process causes the gapto shrink. At least some surfactant particles are disposed at sidewallboundaries of the gap.

The method 600 includes a step 640 of performing a developing process onthe first and second photoresist patterns. The developing processremoves the chemical mixture in the gap and over the photoresistpatterns. The surfactant particles disposed at sidewall boundaries ofthe gap reduce a capillary effect during the developing process.

In some embodiments, the chemical material has thermal cross-linkingproperties such that the baking process of step 630 causes a portion ofthe chemical mixture to become cross-linked with sidewalls of the firstand second photoresist patterns. The cross-linked portions of thechemical mixture define the sidewall boundaries of the gap.

In some embodiments, the chemical material contains thermo-responsivecopolymers that facilitate a flow of the first and second photoresistpatterns during the baking process of step 630. The flow of the firstand second photoresist patterns causes the gap to shrink.

It is understood that additional steps may be performed before, during,and after the steps 610-640 of method 600. For example, the method 600may include a step of, before the coating: mixing the surfactantparticles in the chemical material in a homogeneously distributedmanner. In some embodiments, the method 600 includes a step of, beforethe mixing: obtaining a fluorinated surfactant as the surfactantparticles. In some embodiments, the method 600 includes a step of,before the mixing: obtaining a hydrocarbon surfactant as the surfactantparticles. In some embodiments, the method 600 includes a step of, afterthe developing process is performed: forming a contact hole, wherein theforming of the contact hole is performed using the first and secondphotoresist patterns as a mask. In some embodiments, the method 600includes a step of, after the developing process is performed: forming aphoto-sensitive pixel of an image sensor device, wherein the forming ofthe photo-sensitive pixel is performed using the first and secondphotoresist patterns as a mask.

FIG. 17 is a flowchart illustrating a method 700 of fabricating asemiconductor device according to embodiments of the present disclosure.The method 700 includes a step 710 of mixing surfactant compounds in achemical material that has thermal cross-linking properties.

The method 700 includes a step 720 of forming a first photoresistpattern and a second photoresist pattern over a substrate, the first andsecond photoresist patterns defining a trench therebetween.

The method 700 includes a step 730 of applying the chemical materialhaving the surfactant compounds mixed therein over the substrate andaround the first and second photoresist patterns.

The method 700 includes a step 740 of heating the chemical material andthe photoresist patterns, thereby transforming portions of the chemicalmaterial disposed on the first and second photoresist patterns into across-linking material. The sidewall surfaces of the cross-linkingmaterial have the surfactant compounds disposed thereon.

The method 700 includes a step 750 of developing the first and secondphotoresist patterns by applying a developer solution. The developingreduces a width of the trench by removing portions of the chemicalmaterial that have not been transformed into the cross-linking material.The surfactant compounds disposed on the sidewall surfaces of thecross-linking material reduce capillary forces experienced by the firstand second photoresist patterns during the developing.

In some embodiments, the mixing comprises: mixing the surfactantcompounds uniformly in the chemical material.

It is understood that additional steps may be performed before, during,and after the steps 710-750 of method 700. For example, in someembodiments, the method 700 may include a step of, before the mixing:obtaining fluorinated surfactants as the surfactant compounds. In someembodiments, the method 700 may include a step of, before the mixing:obtaining hydrocarbon surfactants as the surfactant compounds. In someembodiments, the method 700 may include a step of, after the developing:forming a contact hole through an etching process in which first andsecond photoresist patterns serve as an etching mask. In someembodiments, the method 700 may include a step of, after the developing:forming a radiation-sensing region of an image sensor device through anion implantation process in which the first and second photoresistpatterns serve as an implantation mask.

FIG. 18 is a flowchart illustrating a method 800 of fabricating asemiconductor device according to embodiments of the present disclosure.The method 800 includes a step 810 of mixing surfactant moleculesthroughout a chemical material. The chemical contains thermo-responsiveco-polymers that facilitate a flow of a photoresist material in responseto being baked.

The method 800 includes a step 820 of forming a first photoresistpattern and a second photoresist pattern over a substrate. The first andsecond photoresist patterns are separated by a trench.

The method 800 includes a step 830 of coating the chemical material withthe surfactant molecules mixed therein over the first and secondphotoresist patterns.

The method 800 includes a step 840 of baking the chemical material andthe photoresist patterns. The chemical material in response to thebaking induces the first and second photoresist patterns to flow towardeach other, thereby reducing a lateral dimension of the trench. Thechemical material provides mechanical support for sidewalls of the firstand second photoresist patterns during the flow. The surfactantmolecules exist on sidewall boundaries of the trench.

The method 800 includes a step 850 of developing the first and secondphotoresist patterns using a developer solution. The developer solutionremoves the chemical material. Capillary forces are reduced inside thetrench by the presence of the surfactant molecules on the sidewallboundaries of the trench.

In some embodiments, the mixing in step 810 includes mixing fluorinatedsurfactant molecules within the chemical material. In some embodiments,the mixing in step 820 includes mixing hydrocarbon surfactant moleculeswithin the chemical material. In some embodiments, the mixing comprises:mixing the surfactant molecules evenly in the chemical material.

It is understood that additional steps may be performed before, during,and after the steps 810-850 of method 800. For example, in someembodiments, the method 800 includes a step of forming a contact hole atleast in part by an etching process, the first and second photoresistpatterns serving as a mask for the etching process. In some otherembodiments, the method 800 includes a step of after the developing:forming a radiation-sensing region of an image sensor device by an ionimplantation process, the first and second photoresist patterns servingas an implantation mask for the ion implantation process.

Based on the above discussions, it can be seen that the presentdisclosure offers advantages over conventional semiconductorfabrication. It is understood, however, that other embodiments may offeradditional advantages, and not all advantages are necessarily disclosedherein, and that no particular advantage is required for allembodiments. One advantage is that the photoresist patterns can beformed with a high aspect ratio trench in between without risking thecollapse of the photoresist patterns. For example, as discussed above, achemical shrinkage material (either RELACS or SAFIER) allows thephotoresist patterns to effectively “shrink” laterally, which reducesthe lateral dimension or width of the trench. In this process, adeveloper solution used to remove the shrinkage material gives rise tocapillary forces inside the trench, which may pull on the photoresistpatterns. Conventionally, the capillary forces may cause the photoresistpatterns to collapse if the photoresist patterns are too tall and/or ifthey are located too close to one another. To avoid this problem,conventionally photoresist pattern formation methods limit the aspectratio of the trench by reducing the height of the photoresist pattern,or increasing the space in between them, neither of which is desirablein advanced semiconductor fabrication with ever-decreasing device sizes.

In comparison, the present disclosure overcomes this problem by mixingsurfactant particles in the chemical shrinkage material, such that somesurfactant particles are disposed on the sidewall surfaces of the trenchduring the developing process. The surfactant particles reduce thesurface tension, which is correlated with the capillary forces. As such,the effects of the capillary forces experienced by the photoresistpatterns are also reduced by the presence of these surfactant particles,which diminishes the risks of photoresist pattern collapse. As a result,the present disclosure can form photoresist patterns with a greaterheight and/or closer spacing than conventionally possible, while notrisking the photoresist pattern collapse issue. Such photoresistpatterns can be used as a mask in subsequent processes such as contacthole formation or pixel ion implantation to achieve better results, suchas smaller device sizes or greater device pattern density. Otheradvantages may include lower costs, increased yield, and compatibilitywith existing fabrication process flow.

One aspect of the present disclosure involves a method of fabricating asemiconductor device. A first photoresist pattern and a secondphotoresist pattern are formed over a substrate. The first photoresistpattern is separated from the second photoresist pattern by a gap. Achemical mixture is coated on the first and second photoresist patterns.The chemical mixture contains a chemical material and surfactantparticles mixed into the chemical material. The chemical mixture fillsthe gap. A baking process is performed on the first and secondphotoresist patterns, the baking process causing the gap to shrink. Atleast some surfactant particles are disposed at sidewall boundaries ofthe gap. A developing process is performed on the first and secondphotoresist patterns. The developing process removes the chemicalmixture in the gap and over the photoresist patterns. The surfactantparticles disposed at sidewall boundaries of the gap reduce a capillaryeffect during the developing process.

One aspect of the present disclosure involves a method of fabricating asemiconductor device. Surfactant compounds are mixed in a chemicalmaterial that has thermal cross-linking properties. A first photoresistpattern and a second photoresist pattern are formed over a substrate.The first and second photoresist patterns define a trench therebetween.The chemical material having the surfactant compounds mixed therein isapplied over the substrate and around the first and second photoresistpatterns. The chemical material and the photoresist patterns are heated,thereby transforming portions of the chemical material disposed on thefirst and second photoresist patterns into a cross-linking material.Sidewall surfaces of the cross-linking material have the surfactantcompounds disposed thereon. The first and second photoresist patternsare developed by applying a developer solution. The developing reduces awidth of the trench by removing portions of the chemical material thathave not been transformed into the cross-linking material. Thesurfactant compounds disposed on the sidewall surfaces of thecross-linking material reduce capillary forces experienced by the firstand second photoresist patterns during the developing.

One aspect of the present disclosure involves a method of fabricating asemiconductor device. Surfactant molecules are mixed throughout achemical material. The chemical contains thermo-responsive co-polymersthat facilitate a flow of a photoresist material in response to beingbaked. A first photoresist pattern and a second photoresist pattern areformed over a substrate. The first and second photoresist patterns areseparated by a trench. The chemical material with the surfactantmolecules mixed therein is coated over the first and second photoresistpatterns. After the coating, the chemical material and the photoresistpatterns are baked. The chemical material in response to the bakinginduces the first and second photoresist patterns to flow toward eachother, thereby reducing a lateral dimension of the trench. The chemicalmaterial provides mechanical support for sidewalls of the first andsecond photoresist patterns during the flow. The surfactant moleculesexist on sidewall boundaries of the trench. After the baking, the firstand second photoresist patterns are developed using a developersolution. The developer solution removes the chemical material.Capillary forces are reduced inside the trench by the presence of thesurfactant molecules on the sidewall boundaries of the trench.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method, comprising: forming a mixture by mixingsurfactant particles with a chemical material, wherein the surfactantparticles have a material composition configured to reduce a surfacetension; forming a first photoresist pattern and a second photoresistpattern over a substrate, wherein the first photoresist pattern and thesecond photoresist pattern are separated by a trench having a firstaspect ratio; applying the mixture on the first photoresist pattern andthe second photoresist pattern; varying a shape of the first photoresistpattern and the second photoresist pattern by performing a thermalprocess, wherein the trench has a second aspect ratio after theperforming of the thermal process, the second aspect ratio being greaterthan the first aspect ratio; and developing the first photoresistpattern and the second photoresist pattern, wherein a reduction of thesurface tension provided by the surfactant particles prevents a collapseof the first photoresist pattern or the second photoresist patterncaused by capillary forces during the developing.
 2. The method of claim1, wherein the surfactant particles each include a molecule having afirst end that is water-soluble and a second end that is oil-soluble,wherein the second end is opposite the first end.
 3. The method of claim1, wherein the surfactant particles are configured to aggregate to formmicelles.
 4. The method of claim 1, wherein the surfactant particleseach include a fluorinated compound or a hydrocarbon compound.
 5. Themethod of claim 1, wherein the mixing comprises mixing the surfactantparticles uniformly throughout the chemical material.
 6. The method ofclaim 1, wherein the applying the mixture comprises applying thesurfactant particles on sidewall surfaces of the first photoresistpattern and the second photoresist pattern.
 7. The method of claim 1,wherein: the first aspect ratio is greater than about 8:1; and thesecond aspect ratio is greater than about 11:1.
 8. The method of claim1, wherein in response to the performing of the thermal process, thefirst photoresist pattern and the second photoresist pattern eachbecomes taller and wider.
 9. The method of claim 8, wherein in responseto the performing of the thermal process, a cross-linking film is formedaround the first photoresist pattern and the second photoresist pattern,thereby causing the first photoresist pattern and the second photoresistpattern to each become taller and wider.
 10. The method of claim 8,wherein the thermal process is performed using a process temperature ina range between about 140 degrees Celsius and about 170 degrees Celsius,and with a process duration between about 60 seconds and about 120seconds.
 11. The method of claim 1, wherein in response to theperforming of the thermal process, the first photoresist pattern and thesecond photoresist pattern each becomes shorter and wider.
 12. Themethod of claim 11, wherein in response to the performing of the thermalprocess, the first photoresist pattern and the second photoresistpattern flow laterally toward each other, thereby causing the firstphotoresist pattern and the second photoresist pattern to each becomeshorter and wider.
 13. The method of claim 11, wherein the thermalprocess is performed using a process temperature in a range betweenabout 100 degrees Celsius and about 160 degrees Celsius, and with aprocess duration between about 60 seconds and about 90 seconds.
 14. Themethod of claim 1, further comprising: using the trench to define acontact hole.
 15. The method of claim 1, further comprising: using thetrench to define one or more pixels of an image sensor device.
 16. Amethod, comprising: forming a first photoresist pattern and a secondphotoresist pattern over a substrate, wherein the first photoresistpattern and the second photoresist pattern are separated by a trench;coating a shrinkage material over the first photoresist pattern and thesecond photoresist pattern, wherein the shrinkage material includes asurfactant that lowers a surface tension; baking the first photoresistpattern and the second photoresist pattern, wherein in response to thebaking, the shrinkage material causes the first photoresist pattern andthe second photoresist pattern to alter their shapes, thereby enlargingan aspect ratio of the trench; developing the first photoresist patternand the second photoresist pattern, wherein the surfactant of theshrinkage material lessens an impact of a capillary force pulling on thefirst photoresist pattern of the second photoresist pattern during thedeveloping, thereby reducing a likelihood of a collapse of the firstphotoresist pattern or the second photoresist pattern; and using thetrench having the enlarged aspect ratio to define a component of anintegrated circuit device.
 17. The method of claim 16, furthercomprising: before the coating, mixing the surfactant uniformly withinthe shrinkage material, wherein the surfactant includes molecules eachhaving a water-soluble first end and an oil-soluble second end oppositethe first end.
 18. The method of claim 16, wherein in response to thebaking, portions of the shrinkage material coated around the firstphotoresist pattern and the second photoresist pattern transform intocross-linked films that adhere to an upper surface and side surfaces ofthe first photoresist pattern and the second photoresist pattern,thereby enlarging the first photoresist pattern and the secondphotoresist pattern vertically and horizontally and enlarging the aspectratio of the trench.
 19. The method of claim 16, wherein in response tothe baking, the shrinkage material facilitates a flow of the firstphotoresist pattern and the second photoresist pattern toward eachother, thereby enlarging the first photoresist pattern and the secondphotoresist pattern horizontally and enlarging the aspect ratio of thetrench.
 20. A method, comprising: mixing surfactant particles uniformlywithin a shrinkage material, wherein the surfactant particles include asurface-tension-reducing material, and wherein the shrinkage materialincludes a first material that becomes cross-linked in response to athermal process or a second material that facilitates a flow of aphotoresist material in response to the thermal process; forming a firstphotoresist pattern and a second photoresist pattern over a substrate,wherein the first photoresist pattern and the second photoresist patternare separated by a trench; applying the shrinkage material with thesurfactant particles mixed therein onto the first photoresist patternand the second photoresist pattern; baking the first photoresist patternand the second photoresist pattern, wherein in response to the baking,an aspect ratio of the trench increases due to: portions of theshrinkage material becoming cross-linked and adhering to upper and sidesurfaces of the first photoresist pattern and the second photoresistpattern, or the shrinkage material causing the first photoresist patternand the second photoresist pattern to flow laterally; developing thefirst photoresist pattern and the second photoresist pattern, wherein acapillary effect during the developing is reduced by the surfactantparticles of the shrinkage material, thereby preventing a collapse ofthe first photoresist pattern or the second photoresist pattern; andafter the developing, using the trench to define an element of anintegrated circuit device.